The old video below (2014) shows how we manage the digital archaeological drawing on the field with QGIS.
The vector layers can be related to georeferenced photomosaic (bidimensional photomapping), or to georeferenced orthophoto (coming from 3D operations based on SfM/MVSR techniques). Of course orthophoto are the best solution, but currently the 3D work-flow with standard hardware is pretty slow. This is the reason why for almost all the palimpsestic documentation we still work with both the system: 2D photomapping and 3D SfM; depending on the time-table we have on the field, we choose the post-processing operations.
Within QGIS it is possible to draw vector layers in different ways. Basing on our experience (as you can see in the video), the best two best solution are:
1. to use the plugin Freehand Editing (if you want to experience something that is really similar to the old-traditional methodology, with the pencil and the paper)
2. to use the standard vector drawing tools (if you want to avoid too complex sahpes, like polygons with too many nodes)
IMHO, Freehand Editing plugin is a perfect solution for field operation, so that I am planning to add it into qgis-archeos-plugin, for ArcheOS Hypatia.